Power door systems for use on elevators, transit vehicles, etc., which are generally intended for the general public, have been well known in the art prior to the development of the present invention. Such door systems must operate in a rather challenging environment in which the safety of the public may be at stake. Additionally, these door systems are subjected to heavy usage and are generally required to operate very rapidly. Furthermore, the doors in such door systems are often quite massive and must be stopped very rapidly in the event that an obstruction is detected to avoid crushing a person in the path of a door.
An electric motor for such a power door system may be powered by a pulse width modulated H-bridge amplifier. Typically, the motor is a DC motor in which the magnetic field is provided by a permanent magnet. The H-bridge amplifier switches a voltage and current from a pair of power supply lines to the electric motor and selects the polarity of the voltage applied to the motor in order to control the direction of the torque generated by the motor.
The H-bridge amplifier typically operates in one of four modes, which are referred to in the art as "quadrants". These are:
(1) Accelerate the door(s) in the door closing direction during the first portion of a door closing stroke. PA1 (2) Decelerate the door(s) during the final portion of a door closing stroke. PA1 (3) Accelerate the door(s) in the door opening direction during the first portion of a door opening stroke. PA1 (4) Decelerate the door(s) during the final portion of a door opening stroke.
These four modes of operation are generally performed in a closed loop mode. A position encoder provides one or more position signal(s) which continuously indicates the position of the door(s).
The position information is conveyed to a logical device, which may be, for example, a CPU or a programmable logic chip. This logical device generates a signal indicating the appropriate acceleration or deceleration of the door(s). The signal typically is a pulse width modulated signal which is conveyed to the control inputs of the four switching devices of the H-bridge amplifier.
In the event that the door(s) encounter an obstruction, an obstruction detection indicator sends a signal to stop the doors. Stopping the doors may be done in a closed loop mode in which the logical device which controls the motor based on signals from the position encoder sends signals to decelerate the door(s).
One difficulty with this approach is that the kinetic energy of the door(s) is regeneratively converted to electrical energy which is fed back to the power distribution line which provides power to the doors. Since it is necessary to stop the doors very quickly in the event of an obstruction, this regenerative energy is fed to the power distribution line during a very short time interval and causes a spike of voltage in the power distribution line. This may cause the entire door system to fail.
Another difficulty is that oscillations may occur, the velocity of the door(s) overshooting zero speed and changing polarity. This requires a major intervention by the logical device to damp the resulting ringing of the loop dynamics as the speed decays to zero.
Another difficulty with this approach is that it is not fail safe. If the logical device malfunctions when the door(s) are in motion, the doors will continue to move and may injure a person in their path. Likewise, if the position encoder fails, the required deceleration of the door(s) will not occur.
Another approach to the problem of absorbing the kinetic energy of the door(s) when an obstruction is detected is to short the motor through a shunt resistor so that the kinetic energy of the door(s) is absorbed as heat in the resistor. This is an open loop method which does not require functioning of the encoder or the logical device.
This approach has two principal difficulties. One is that the door(s) do not decelerate at a constant rate. The current through the resistor and the motor is proportional to the emf of the motor, which is proportional to its speed. Since the emf of the motor decreases as its speed decreases, the current and hence the motor torque decrease. The maximum allowed deceleration of the door is limited by the strength of the door drive hardware which connects the motor to the door(s), and may also be limited by electrical considerations. Since, with the shunt resistor approach, the deceleration decreases below the maximum allowed deceleration, the distance required to stop the door(s) is greater than if the maximum allowed deceleration is maintained during the stopping event. Such doors are more likely to cause harm to a person than doors which are stopped with a motor current which remains constant through most of the stopping event, because the latter doors stop in a shorter distance.
Another disadvantage of the shunt resistor approach is that if the shunt is located in the door control enclosure, its heat is generated in the enclosure and may cause overheating of the electronic components.
A further disadvantage of the shunt resistor approach is that as the motor ages, its magnetic field becomes weaker. The emf generated by an aged motor at any speed is therefore lower than when it was new. It, therefore, generates less current at any speed and the door is not decelerated as rapidly and the effect on a person in the path of the door is greater.